Effect of Molecular Weight on Thermoelectric Performance of P3HT Analogues with 2-Propoxyethyl Side Chains

By replacing hexyl chains in poly(3-hexylthiophene) (P3HT) with 2-propoxyethyls, four poly(3-(2-propoxyethyl)thiophene) (P3POET) homopolymers with comparable polydispersity indexes (PDI) and regioregularities were prepared herein in addition with step increment of about 7 kDa on number-average molecular weight (Mn) from around 11 to 32 kDa (accordingly denoted as P11k, P18k, P25k, and P32k). When doped in film by FeCl3 at the optimized conditions, the maximum power factor (PFmax) increases greatly from 4.3 µW·m−1·K−2 for P11k to 8.8 µW·m−1·K−2 for P18k, and further to 9.7 µW·m−1·K−2 for P25k, followed by a slight decrease to 9.2 µW·m−1·K−2 for P32k. The close Seebeck coefficients (S) at PFmax are observed in these doped polymer films due to their consistent frontier orbital energy levels and Fermi levels. The main contribution to this PFmax evolution thus comes from the corresponding conductivities (σ). The σ variation of the doped films can be rationally correlated with their microstructure evolution.